1. Field of the Invention
The present invention relates to a three-phase motor which can be fed from a converter. More particularly, the present invention includes a shielding device that eliminates capacitive current in the stator slots so as to prevent a current circuit in the laminated core of the motor and avoid inducing voltage in the motor shaft.
2. Description of the Related Art
Bearing damage is on the rise in modern converter-fed three-phase motors due to the high switching speed of IGBT converters. This bearing damage is caused by high-frequency bearing currents flowing through the shaft, bearing and laminated stator core. The bearing currents are induced because a high voltage gradient of the feed voltage with respect to ground induces a voltage in the motor shaft, which results in bearing currents through the bearings and the housing. As remedial measures to prevent these bearing currents, the person skilled in the art is familiar with insulated bearings composed of ceramic balls or with an insulating oxide layer on the outer ring of the bearing. These insulated bearings are relatively expensive. The person skilled in the art is also aware that it is possible to use inductors or filters to reduce the voltage gradient du/dt at the converter output. However, with this design standardization is difficult since it is necessary to define the type and cable length of the motor.
The object of the present invention is to efficiently prevent bearing damage in three-phase motors in an easy and economical manner.
The present invention solves the aforementioned problem without major reliance on parameters of the motor. As a result, the current carrying properties of the converter or cables thus have no influence on the bearing currents. Standardization becomes readily possible and efficient, irrespective of the cable length, motor type or the like. According to the present invention, the capacitive outgoing current, which, according to the prior art, flows out of the winding of the three-phase motor in the laminated stator core of the three-phase motor, is carried away by a conductive shield in each stator slot between the winding and the stator to the ground connection. In this case, this shield is connected to the housing (i.e., to PE or ground) on only that side of the stator on which the three-phase current is fed into the winding of the three-phase motor. This connection may be configured in such a way that the shield is connected via a shield connection from the terminal box to the housing (PE) for grounding. Alternatively, the shield can be connected to the laminated core on the side of the stator on which the three-phase current is fed in. This connection of the shield to the laminated core can be achieved particularly efficiently by removing the insulation between the shield and the laminated core on this side, so that the shield makes direct conductive contact with the laminated core on this side.
According to the present invention, prevention of bearing current operates so that the capacitive outgoing current, whose magnitude is not changed by the invention, does not flow in the laminated stator core. Rather, the capacitive outgoing current flows in the shield, and is fed back through the slot to the feed point. As a result, the capacitive current in the slots is cancelled, there is no longer any circular flow in the laminated stator core and thus no voltage is induced in the motor shaft, which is responsible for the bearing current that damages the bearings.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.